SYSTEMATIC STUDY OF INTERSTELLAR ICES IN NEARBY GALAXIES

The absorption features due to interstellar ices, especially $H_2O$ and $CO_2$ ices, provide us with crucial information on present and past interstellar environments, and thus the evolutionary histories of galaxies. Before AKARI, however, few detections of ices were reported for nearby galaxies. The AKARI's unique capability of near-infrared spectroscopy with high sensitivity enables us to systematically study ices in nearby galaxies. Thus we have explored many near-infrared spectra ($2.5-5{\mu}m$) of the 211 pointed observations, searching for the absorption features of ices. As a result, out of 122 nearby galaxies, we have significantly detected $H_2O$ ice from 36 galaxies and $CO_2$ ice from 9 galaxies. It is notable that the ices are detected not only in late-type galaxies but also in early-type galaxies. We find that $CO_2$ ice is more compactly distributed near the galactic center than $H_2O$ ice. Finally, we suggest that the gas density of a molecular cloud and UV radiation may be important factors to determine the abundance of ices.